Apparatus and process for forming an optical fibre covered by a metallic sleeve

ABSTRACT

The optical fibre to be protected is dipped into a bath of molten metal which solidifies on contact with the fibre and forms the desired sleeve. The fibre passses through dies immersed into the bath and of which the spacing is adjustable so as to vary the distance along which the fibre is in contact with the molten metal and, consequently, the thickness of the sleeve.

The invention relates to a process for covering an optical fiber with ametallic sleeve, particularly an aluminum sleeve.

It is known that it is desirable to protect the optical fibers fromcorrosion caused by atmospheric agents which gradually impair theirlight transmission properties and their resistance qualities withrespect to mechanical forces. An aluminum sleeve of a thickness of from10 to 150 μm, preferably deposited by Passing the fiber through meltedaluminum which adheres to the fiber and solidifies on contact,constitutes a good protection.

Thus the J-57 106.542 (FURUKAWA) text describes the stripping of opticalfibers (for example, by burning their protective organic-resin sleeve)and covering them with aluminum by passing them through a bath of thismelted metal.

The J-57 071.840 (FURUKAWA) text describes the covering of opticalfibers with metals by means of an extrusion process according to whichthe covering metal (Al-Cu alloy) is maintained in a semi-melted phase(in which the proportion of the solid phase is 80-90%). In this case,the temperature at which the fiber is put in contact with the meltedmetal is reduced as much as possible, since this elevation oftemperature has a negative influence on its mechanical qualities.

The J-58 074.543 (FUJITSU) text discloses the circulating of an opticalfiber in a melted-metal bath (Sn, Al, etc.), with the object of coveringit by depositing this type of a metal, the thus covered fiber beinginstantly cooled by sprinkling with cold water or by a gas current.

The J-58 027.977 (FURUKAWA) document describes the plating offilamentous materials, particularly optical fibers. According to thisprocess, a heatable tube is placed on the surface of a melted-metal bathcontained in a double-walled crucible; the metal is heated above itsmelting point, and the tube is heated to a temperature that is slightlyabove that of the melted metal; then the filament is circulated in thetube while the pressure is reduced there in order to promote anevaporation in a vacuum. Thick metallic covering layers are obtained bymeans of this technique.

The J-58 045.133 (FUJITSU) text discloses the depositing of metallicfilms on optical fibers as soon as they are drawn. The used metalscontain Al and Sn. Cu prevents the oxidizing of the metal by actingunder the protection of an inert gas, such as nitroqen or arqon. Thethus obtained fibers exhibit low losses with respect to the transmissionand a high tensile strength.

The J-58 045.134 (FUJITSU) text describes the covering of optical fiberswith metals, in which case the used process includes putting the fiberscoming out of the drawing furnace in contact with a current of hot gas.This gas current, which is applied to the fibers while the coveringmetal is still liquid, makes it possible to control the thickness andthe uniformity of the covering and results in fibers with low losseswith respect to transmission.

The J-59 035.046 (FUJITSU) describes the application of a metalliccovering to optical fibers by passing them through a nose piece ornozzle containing melted metal and subjecting it to ultrasonics. Thistreatment is suitable for freshly drawn fibers and furnishes adheringmetallic films which can be welded, which permits the connecting of thefibers among one another.

The J-60 194.041 (SUMITOMO) text teaches the covering of optical fiberswith a metallic film by passing them, under a reduced pressure, throughmelted metal, the temperature of the fiber being lower than or equal tothat of the melted metal. After the depositing of the melted metal, thefiber is passed through a wire gauge.

Although the techniques of the prior art permit the solving of a largenumber of problems concerning the plating of optical fibers with meltedmetal, certain problems remain nevertheless, or rather, certainsuggested solutions are difficult to apply and should be simplified. Inparticular, the control of plating conditions is always difficult toimplement as well as the uniformity of the thickness and the regularityof the deposit. It is particularly difficult, in view of the respectivetemperatures of the fiber and of the melted metal, to ensure a goodcontacting time between these two elements so that the metal adhereswell to the fiber and solidifies there in the form of a homogeneoussleeve of an even thickness.

The process of the present invention, as summarized in claim 1,constitutes a step toward improving and simplifying the technique ofcovering quartz or glass fibers with a protective metallic sleeve.

A device, which permits the implementation of the present process, isdescribed in claim 11.

An embodiment of this type of a device is shown, in the form of aschematic section, in the attached drawing to which reference is madeaccording to the description.

FIG. 1 represents a crucible provided with vertical dies for covering anoptical fiber with a protective metallic sleeve.

FIG. 2 is an enlarged detail of one of the dies of the crucible of FIG.1.

The device of FIG. 1 consists of a crucible 1 delimited by a bottom 2,walls 3 and a cover 4. This crucible stands on a horizontal table 5which can be moved longitudinally and transversely by a play ofmicrometrical screws 6 and 7. The bottom of the crucible is pierced andtapped for receiving a threaded die 8, the truncated nose piece 9 ofwhich penetrates into the interior of the crucible 1. This is filledwith melted aluminum 10.

The crucible contains another die 11 disposed in the cover 4 and axiallyopposite the nose piece 9 of the die 8. By means of the play of thescrew thread of the latter, the level of the nose piece 9 can beshifted, i.e., the depth h at which it is dipped in the melted metal aswell as the distance G between the flat parts of the nose pieces of thetwo dies; the die 8 can be adjusted until it touches the die 11 (G=0).

The present device also contains a heating winding 12 for maintainingthe aluminum in a liquid state. Furthermore, a drawing system for anoptical fiber (not shown, but conventional) furnishes an optical fiber13 which can be circulated throuqh the dies 8 and 11 and wound on astorage attachment which is shown schematically by means of a drum 14.It will be noted that, in order to avoid that the just drawn fiber comesin contact with air, the fiber can be protected by means of a dry orinert gas, such as argon.

FIG. 2, at a very enlarged scale, shows the mouth of the nose piece 9 ofthe receiving die during the operation of the covering device. In brief,the metal is melted in the crucible and maintained in the melted stateat a temperature provided by means of the heating device 12. A fiber 13is drawn, for example, from a rod preform (not shown) and is circulatedin the dies 8 and 11 with the inside diameter D, the die 8 havingPreviously been screwed in completely in order to touch the die 11.Then, after having carefully centered the fiber by means of the play ofthe adjusting devices 6 and 7, the die 8 is gradually unscrewed until aspace G is created between the dies, in such a manner that the meltedmetal comes in contact with the fiber 13 having the diameter d whichmoves in the direction of the arrow. This movement, in the metal that isin the melted state, creates a meniscus 15 in the form of a "vortex"orof a funnel, the edge 16 of which fits the mouth of the nose piece 9 andthe rod 17 creates and lengthens the protective sleeve 18 of solidifiedmetal which surrounds the fiber 13 at the outlet of the die 8. Thegenerating line 19 of this meniscus in the form of a "vortex"forms avariable curve, the minimum radius R of which relates to an annularregion 20 placed at a distance r from the axis of the system.

These parameters are interesting for the following reasons: In order toavoid that the melted metal leaks between the fiber and the walls of theguide of the die 8 (or in other words, in order to form a protectivesleeve of solidified metal that is straight and uniform), it isnecessary, according to the laws of surface tension, that thehydrostatic pressure ρ gh, which prevails at the level of the nose piece9, does not exceed the value given by the relation σ (1/r+1/R).

It may be admitted that R is the middle between D/2 and d/2. Forexample, if d amounts to 100 μm (0.1 mm) and D=0.4 mm, r is close to 0.1mm. For a circulating speed of the fiber of 30 m/min. (50 cm/sec.), R isapproximately 0.2 mm, resulting in 1/r+1/R=150 cm-⁻¹, and, in the meltedaluminum (ρ =2.7; σ=914 d/cm), h≦914×150/2.7×981≃52 cm. The advantageousoperating depth in the melted aluminum is therefore 1.5 to 10 cm.However, h may be higher or lower than this range.

It must be pointed out, however, that this type calculation must becarried out while taking into account the dynamic factor created by themovement of the fiber itself.

A "pressure"of this type is given by the relation ρv² /2 wherein v isthe speed of the fiber in cm/sec. In the above-mentioned case, thefollowing will therefore exist: h=v² /2 g=1.3 cm, which is negligible inproportion to the above-calculated values of h.

Generally, it was found that, for covering an optical fiber of a gaugeof 50 to 200 μm, a die must be used, the inside diameter of which isunderstood to be between 200 and 1,000 μm. The operation advantageouslytakes place at a depth h of 2 to 10 cm. By varying G, the distancebetween the truncated flat parts of the dies, between apProximately 0.5and 5 mm, the thickness of the aluminum sleeve may be adjusted betweenapproximately 10 and 150 μm.

It should be pointed out that if, in principle, it is advantageous toleave as little play as possible between the fiber and the guide of thedie, the distance D-d being 50-750 μm and it should not be less thanapproximately 20-100 μm; in fact, below that, the centering of the fiberis extremely difficult.

Furthermore, the possible tendency of the metal to form leaks andirregularities in the die 9 can be counteracted (in the event that asignificant play is arranged between it and the fiber) by applying a lowgas pressure upstream of the die 11 or an excess pressure downstream ofthe die 8, the crucible then being part of a tight chamber containing anupper compartment (where the fiber is delivered) and a lower compartmentwhere the covered fiber is stored. The work may take place in an inertatmosphere (nitrogen, argon, etc.) or in air, the excess pressurerelating to the upper compartment and the low pressure relating to thelower compartment. Pressure variations of from 0.05 to 0.5 bar areappropriate.

In the present process, the preheating of the fiber before the platingis not critical. Generally, when the drawing station is located at asufficient distance from the crucible, for example, between 0.5 to 2 m,the fiber, by means of cooling, acquires a temperature that is suitablefor its covering by means of the aluminum. If this is not the case, forexample, when the fiber is not drawn in situ, a preheating to 300°-400°C. may be provided before the plating. Preferably, the temperaturedifference Δ T between the fiber and the melted metal is between 100°and 650° C.

The temperature of the melted aluminum must be controlled within certainlimits. It may vary, for example, between 660 and 750° C., but theresults are not identical for all values within this range. At lowertemperatures, the solidifying may be too fast and the deposit may becomeirregular or even block the passage of the fiber. At temperatures thatare too high, the covering is too thin or has a tendency to run. Thepreferred temperature range is between 663° and 690° C.

The present process applies to the covering of optical fibers made ofquartz or another transparent mineral of a fixed or radially variableindex of refraction. It is also applicable to other mineral fibers in sofar as their mechanical resistance is sufficient at the temperatures ofthe melted metal bath.

In addition, other metals are also suitable in the melted state, such asSn, Pb, Cu, Ni, ag, etc., as well as their alloys with or withoutaluminum. The operating parameters can, of course, be modified accordingto the melting temperature and the physical properties (viscosity,surface tension) of each considered metal or alloy. The filling of thecrucible may be carried out discontinuously or continuously. In thelatter case, the supplying of the metal takes place in the form of awire, the progressive melting of which adjusts the metal deposited onthe fiber and the output of which maintains the level of the liquidconstant in the crucible.

It will be noted that in the construction of the dies shown as anembodiment in the drawing, the width L of the flat portion of thefrustrum of the cone of the dies may be between approximately 0.5 and 5mm; the angle of the slope of the truncated portion may be betweenapproximately 40° and 75° . In order to cool the fiber at the outlet ofthe receiving die 8, ambient air or a jet of cooling air or of coolingliquid may be used.

The following examples illustrate the invention.

The operation was carried out by means of a unit consisting of a stationfor drawing glass fibers from a preform, i.e., a HERASIL-I-type silicarod (Heraeus, Wisag), of a diameter of 12 mm and a length of 50 cm whichwas drawn by means of a conventional model induction furnace (drawingtemperature 2,000° to 2,200° C.).

For the plating, a graphite crucible was used, like the one shown in thedrawing (placed at a distance of 1.20 m from the drawing station)containing 250 g of aluminum. The dies 8 and 11 are machined of aspecial ceramic material of the "MACOR"type which does not absorb themelted aluminum and is not corroded by it. The upper die is fixed; thelower die 8 can be adjusted with respect to height (by varying G betweenzero and approximately 10 mm), as a result of its screw thread; this diecan be turned by acting upon the hexagonal head 21. The interior guideof the dies has a diameter of 0.5 mm.

First, the dies are brought together until they touch one another bymeans of their plane face; then the aluminum is melted (power of furnace12=4 KW); and then its temperature is adjusted. The fiber is then passedinto the die at the selected speed and is rolled onto the drum 14, thefiber supporting itself on a pulley which is not shown in the drawing.Then the dies are separated by unscrewing the die 8 to a selected valuein such a manner that the melted aluminum is deposited on the fiber andsurrounds it as the desired sleeve. In order to interrupt the formationof the sleeve, the dies are brought together again. In general, in viewof the speed of the drawing of the fiber, the time period during whichit is in contact with the melted metal between the dies is from 10⁻² to10⁻⁴ seconds.

The following table summarizes the operating parameters used for aseries of tests numbered 1 to 7 as well as the obtained results. Thevalues of the table concern, in the order listed: the No. of the test;the diameter in μm of the fiber subjected to the covering; the speed ofits drawing in m/min.; the value in mm of the distance G between thedies; the contact time, in milliseconds, between the fiber and themelted metal; the temperature of the latter in °C.; the thickness of thesleeve in μm; and the breaking load of the covered fiber in N/mm².

    ______________________________________                                        1   130       30    1.87   3.7 666-669  20  2,044                             2   130       30    1.87   3.7 661-662  20  2,414                             3   135       50    2.25   2.7 673      20  1,364                             4   180       30    2.25   4.5 667      20  1,360                             5   130       50    2.25   2.7 663      20  1,580                             6   134       30    1.87   3.7 676      17  2,032                             7   120       30    4.5    9.0 685      15  2,978                             ______________________________________                                    

In addition, the thus covered fibers were subjected to a corrosion testby rolling them on mandrels (diameter 6 and 9 mm) and by subjecting themfor 10 days to a 60° C. temperature in a moist atmosphere at the ratioof 90% (relative humidity). After this time, the controls were allbroken, whereas the protected fibers had resisted the corrosion.

I claim:
 1. A process for depositing on a freshly drawn optical fiber ofthe diameter d a protective aluminum metal sleeve, according to whichthis fiber, in a symmetrically centered manner, is passed through twodies with a truncated nose piece partially wetted by melted metal, oneof which being the supplying means and the other one being the receivingmeans, and being vertically and coaxially superposed opposite oneanother and immersed in a melted-aluminum bath at a distance from oneanother that is sufficient for having the melted metal come in contactwith the fiber in the space located between the plane faces of the nosepieces of the dies and solidifying by contact with the fiber to formsaid protective sleeve, characterized in that, since the dies areaxially movable with respect to one another, they are first reunited bymeans of their plane face, then the metal is liquidified, the fiber isset into circulation through the dies at a speed of 5 to 200 m/min. andthese dies are progressively separated from one another until thedistance between the dies, a value G is between 0.5 and 5 mm, thedistance D-d between the fiber and the bore of the dies being 50 to 750μm, such that said sleeve is formed, the thickness of the sleeve being10 to 150 μm.
 2. A process according to claim 1, the melted aluminumforming, on contact with the fiber, a meniscus and funnel the edge ofwhich is at the same level as the mouth of the nose piece of thereceiving die and the lower end of which, where the solidifying of thesleeve takes place, is joined to the latter, characterized in that, inorder to avoid leaks of the melted metal at the outlet of the receivingdie, the distance r between the axis of the die and the annular zonewhere the meniscus has the most pronounced curve is smaller than theradius of the curve R of this zone.
 3. A process according to claim 2,characterized in that the mouth of the receiving die is immersed in themelted metal at a depth h of 1.5 to 10 cm.
 4. A process according toclaim 1, characterized in that the temperature of the melted aluminum ismaintained between 660° and 750° C.
 5. A process according to claim 4,characterized in that thickness of the covering sleeve varies as afunction of the temperature of the aluminum bath and of the distance Gbetween the dies.
 6. A process according to claim 4, characterized inthat the temperature difference Δ T between the fiber which comes incontact with the melted metal and the metal is between 100° C. and 650°C.
 7. A process according to claim 1, characterized in that the opticalfiber is drawn from a rod preform.
 8. A process according to claim 1,characterized in that the gas pressure around the fiber in thedelivering die is lower than that existing in the receiving die.
 9. Aprocess according to claim 1, characterized in that the melted metalbath is fed continuously by a wire of this metal which progressivelypenetrates the bath in proportion to its melting.
 10. A processaccording to claim 1, characterized in that, at the outlet of thereceiving die, the fiber is cooled and, for the purpose of storage, iswound onto a mandrel.
 11. A Device for depositing a protective aluminumsleeve on a freshly drawn optical fiber comprising the followingelements,(a) a graphite crucible filled with aluminum metal and providedwith devices for melting this aluminum and maintaining it in a meltedstate, this crucible having a bottom and a pierced cover adapted toreceive dies, which respectively receive and deliver an optical fiberwhich moves through them, (b) a horizontal support of the crucible inthe form of a table which can be moved longitudinally and traversly bymicrometrical devices and permits a positioning of the crucible, (c)dies with a truncated, a frusto-conical nose piece, made of a materialthat cannot be absorbed or can be only partially wetted by the meltedmetal, fitting into the holes of the bottom and of the cover of thecrucible respectively, in such a manner that they are axially oppositein the interior of it, at lease one of them being able to be axiallymoved with respect to the other one until it touches it, (d) devices forproducing, by means of hot-drawing, the formation of an optical fiber,devices for circulating this fiber through said dies such that, in thespace between them, it comes in contact with the melted metal, and bymeans of the solidifying of this metal, it is covered by a protectivelayer, and devices for cooling the fiber thus plated and for winding iton a support.
 12. A device according to claim 11, wherein the devicesfor maintaining the metal in a melted state in the crucible areconstituted by an induction furnace.
 13. A device according to claim 11,characterized in that the material of the dies is a machinable ceramichaving the tendency to completely or partially repel the melted metal.14. A device according to claim 11, characterized in that the angle ofthe slope of the frustrum of the cone of the dies is between 40° and75°.